1. Field of the Invention
The present invention relates to an actuator for use in an anti-skid system for wheeled vehicles.
2. Description of the Related Art
A conventional system of this type is disclosed in, for example, U.S. Pat. No. 3,722,960. As described therein, in such a system, high fluid pressure in an accumulator is supplied to a control piston when the wheels of a vehicle are normally rolling, and is excluded from the control piston only when the wheels are likely to lock up.
During operation, vehicles are under non-braked conditions for far less time than under braked conditions. It is for this reason that a large pressure load is applied to seal members arranged in the control piston to form a pressure line leading from the accumulator to the control piston. As a result, the seal members need be replaced after a short period of use due to deterioration and the maintenance of the system is cumbersome.
In a first approach to overcoming the foregoing problem, it was proposed to provide a changeover valve having a first valve arrangement for controllably communicating a hydraulic chamber supplying the fluid pressure to the control piston with the accumulator and stopping such communication and employing a second valve arrangement for controllably communicating the hydraulic chamber with a drain line and stopping such communication. Communication of the hydraulic chamber with the accumulator and the drain line respectively is effected in response to depression and release of a brake pedal. The second valve arrangement may also act as a relief valve to maintain the hydraulic chamber at a relatively low pressure level upon depression of the brake pedal.
It has also been proposed to use a regulator valve including first and second valve arrangements having the same purposes as discussed above and a piston operable under fluid pressure generated by a master cylinder to open and close the first and second valve arrangements. With this regulator valve, the fluid pressure is supplied from the accumulator to the hydraulic chamber in direct proportion to the fluid pressure generated by the master cylinder. The second valve arrangement may also act as a relief valve to maintain the hydraulic chamber at a relatively low pressure level upon depression of the brake pedal. Alternatively, the regulator valve may be provided with a spring for supplying a predetermined minimum pressure to the hydraulic chamber from the accumulator.
When a pump is driven by an electric motor, the accumulator could continuously be operated even after fluid pressure in the accumulator reaches a predetermined upper limit due to a failure in an electronic control circuit adapted to control the electric motor or a pressure-responsive switch adapted to detect the fluid pressure in the accumulator. This results in damage to the pressure line leading from the pump to a third valve and thus, leads to highly dangerous situations. To this end, it is necessary to provide a relief valve in the pressure line as a safety valve so as to prevent the fluid pressure from exceeding a predetermined upper limit. However, provision of such a relief valve may require a large and cumbersome system arrangement and assemblage.